Milky Way Galaxy
200 billion stars
28,000 light years
100 billion galaxies in the observable Universe
10 day exposure photo!
Over 1500 galaxies
in a spot 1/30 the
diameter of the Moon
Farthest and oldest
objects are 12-13 billion
light years away!
Proxima Centauri (Alpha Centauri C)
Closest star (4.2 light-years from the Sun)
Voyager 1: 12 light-hours
from the Sun (90 AU)
Launched in 1977
The most distant humanmade object in space
How can we learn about the life of stars??
• Our life span is ~ 80 years
• Human civilization exists ~ 5000 years
• Our Sun exists at least 4.6 billion years!
Star Clusters – “School Classes”
for Stars
They consist of stars of the same age !
Open clusters
100’s of stars
Globular clusters
100,000 of stars
Giant molecular clouds – stellar nurseries
Great Orion Nebula
The Horsehead Nebula
Star Forming Region RCW 38
Coldest spots in the universe:
T ~ 10 K
Composition:
• Mainly molecular hydrogen
• 1% dust
Protostars: warm clumps of gas surrounded by infalling matter
Disks: planet formation?!
• The matter stops falling on the star
• A star becomes hot enough to sustain the pressure of gravity
Contraction stops when the gravity is
balanced by thermal pressure
Stars are held together by gravity. Gravity tries to compress
everything to the center. What holds an ordinary star up and prevents
total collapse is thermal and radiation pressure. The thermal and
radiation pressure tries to expand the star layers outward to infinity.
Surface temperature 6000 K
Temperature at the center 14,000,000 K!
A puzzle: the Sun and other stars radiate away huge
amounts of energy. They should lose all their heat in
less than a million years!
However, the Sun lives 4.6 billion years
There must be an internal energy source:
nuclear fusion reactions
“Planetary” model of atom
Proton mass: 1.7x10-27 kg
Electron mass: 9x10-30 kg
Nuclear reactions
• Fission: decay of heavy nuclei into lighter fragments
•Fusion: synthesis of light nuclei into a heavier nucleus
A star will live until all hydrogen is exhausted in its core
Our Sun will live 5 billion years more
star mass
(solar masses)
60
Lifetime
(years)
3 million
30
11 million
10
32 million
3
1.5
1
370 million
3 billion
10 billion
1000's
billions
0.1
What happens when all hydrogen is
converted into helium in the core??
Mass defines the fate of the star
Fate of the collapsed core
White dwarf if the remnant is below the
Chandrasekhar limit 1.4 solar mass
 Neutron star if the core mass is less than ~
3 solar masses
 Black hole otherwise

“All hope abandon, ye who enter here”
Dante
Death of Stars
Outer layers expand due to radiation pressure from a hot core
• Surface temperature drops by a factor of ~ 2
• The radius increases by a factor of ~ 100
• Luminosity increases ~ R2 T4 ~ 100-1000 times
The star becomes a Red Giant
In only about 200 million years it will be way too hot for humans
on earth. And in 500 million years from now, the sun will have
become so bright and big, our atmosphere will evaporate, the
oceans will boil off, and surface dirt will melt into glass.
p. 192
p. 193
p. 193
p. 193
p. 193
What is left??
A stellar remnant: white dwarf, composed
mainly of carbon and oxygen
White dwarf
It is extremely dense
All atoms are smashed and the
star is supported by pressure of
free electrons
White Dwarfs
Degenerate stellar remnant (C,O core)
Extremely dense:
1 teaspoon of WD material: mass ≈ 16 tons!!!
Chunk of WD material the size of a beach ball
would outweigh an ocean liner!
White Dwarfs:
Mass ~ Msun
Temp. ~ 25,000 K
Luminosity ~ 0.01 Lsun
As it cools, carbon crystallizes into diamond lattice.
Imagine single diamond of mass 1030 kg!
Don’t rush, you would weigh 15,000 tons there!
White dwarfs in a globular cluster
Death of a massive star
(SLIDESHOW MODE ONLY)
The iron core of a giant star cannot sustain the pressure
of gravity. It collapses inward in less than a second.
The shock wave blows away outer layers of a star,
creating a SUPERNOVA EXPLOSION!
For several weeks the supernova outshines the whole galaxy
Eta Carinae: will explode soon
Distance 7500 ly
Supernova Remnants
Xrays
The Crab Nebula:
Remnant of a
supernova
observed in a.d.
1054
Cassiopeia A
Optical
The Cygnus Loop
The Veil Nebula
Crab nebula: the remnants of supernova 1054
Formation of Neutron Stars
Compact objects more massive than the
Chandrasekhar Limit (1.4 Msun) collapse further.
 Pressure
becomes
so high that
electrons and
protons combine to
form stable neutrons
throughout the
object:
p + e-  n + ne
 Neutron
Star
Properties of Neutron Stars
Typical size: R ~ 10 km
Mass: M ~ 1.4 – 3 Msun
Density: r ~ 1014 g/cm3
 Piece
of
neutron star
matter of the
size of a
sugar cube
has a mass
of ~ 100
million tons!!!
Neutron stars have been theoretically predicted in 30s.
Landau, Oppenheimer, Zwicky, Baade
Isolated neutron stars are extremely hard to observe
However, there are two facts that
can help:
Neutron stars should rotate extremely fast
due to conservation of the angular
momentum in the collapse
 They should have huge magnetic field due
to conservation of the magnetic flux in the
collapse

Discovery of pulsars:
Bell and Hewish, 1967
Jocelyn Bell
When the core is too massive, nothing can
prevent collapse into a black hole
Schwarzschild radius: event horizon for a nonrotating body
2GM
Rs 
2
c
To make a black hole from a body of mass M, one
needs to squeeze it below its Schwarzschild’s radius
Rs
Gravitational collapse:
the body squeezes
below its event horizon
Black holes are NOT big cosmic bathtub drains!
Approaching a black hole R  Rs
(strong field): gravity pull very strong
Far from a black hole R >> Rs
weak field
If our Sun collapses into a black hole, we won’t
see any difference in the gravitational pull (but it
will be VERY cold)
How to observe a black hole if it
does not emit any radiation?

Good news: most stars are in binary systems
The Universe in gamma-ray eyes
Gamma-ray bursts
Gamma-ray bursts
Our Earth and our bodies are made of atoms
that were synthesized in previous generations
of stars